1. Field of the Invention
The invention relates generally to shear wave logging tools for examining earth formations adjacent a borehole and, more particularly, but not by way of limitation, it relates to an improved form of downhole shear wave logging tool and the associated detector devices.
2. Description of the Prior Art
Continuous logging devices utilizing compressional waves in a borehole are quite old and well-known and that type of velocity information is very useful in its own right; however, the actual measurement of shear wave velocities in formations adjacent a borehole has proven much more difficult such that logs of shear velocity are uncommon. The present method for carrying out shear wave logging uses the converted Rayleigh wave which is formed at the hole boundary near the compressional wave transmitter, and processing is included to measure the Rayleigh transit time down the wall of the borehole by measuring time differences between Rayleigh waves reconverted to compressional waves at each of two compressional wave receivers. Various prior art forms of apparatus are known for carrying out this type of wave evaluation. Such utilization of the Rayleigh wave arrival time is utilized since it is large amplitude and arrives later than the shear SV wave which may be buried in noise. Thus, attempts at shear wave velocity which is less than the shear SV wave velocity but not by a reliably constant proportion.
Some prior attempts have been made to generate and detect a shear wave within the borehole wall thereby to avoid those inconsistencies introduced by the borehole fluid. U.S. Pat. No. 3,061,037 in the name of Evans generated shear waves within a borehole by cementing an electrical shear wave generator in the borehole. This approach has obvious drawbacks as regards the desirable practice of traversing the borehole or positioning the logging tool adjacent selected formations. U.S. Pat. Nos. 3,390,377 and 3,333,238 represent those forms of shear wave logging apparatus wherein intermediate wave transit in the fluid of the borehole is utilized and data processing then attempts to ascertain the actual shear wave velocity from the series of seismic waves detected at varying velocities. These teachings transmit through the fluid to propagate shear wave, compressional and Rayleigh wave energy through the formation whereupon energy is detected after once more passing through the fluid to the receivers. Finally, the U.S. Pat. No. 3,213,358 in the name of Piety is of interest as it teaches a method for electromagnetically inducing shear waves in adjacent formations for subsequent detection and velocity analysis.